The first part of the message is because you are giving an initial guess x0 to solve(). This x0 is being ignored because it is not useful to the solver algorithm. The algorithm autogenerates its own starting point.
The second part "Problem is unbounded" is because the solver has determined that there is no finite solution. This is because you asssigned your constraints to "prob" instead of to "Prob".
x = optimvar('x',2,'LowerBound',[0 0],'UpperBound',[inf inf]);
obj = (45* x(1) + 80* x(2));
A = 5*x(1) + 20*x(2) <= 400;
B = 10*x(1) + 15*x(2) <=450;
Prob = optimproblem('Objective',obj, 'ObjectiveSense', 'maximize');
Prob.Constraints.con_1 = A;
Prob.Constraints.con_2 = B;
x0.x = [0 0];
[sol, fval, exitflag, output] = solve(Prob, x0);
